Bone mineralization and growth in term infants fed soy-based or cow milk-based formula

The purpose of this study was to evaluate the adequacy of a soy protein-based formula versus a cow milk protein-based formula for body growth and skeletal mineralization in the first year of life. Thirty-six healthy, term, appropriate for gestational age infants were assigned randomly and prospectively to one of two feeding groups and observed longitudinally over the first year of life. Group 1 infants (n = 18) were fed soy protein isolate-based formula (Isomil with Iron); group 2 infants (n = 17) were fed a cow milk protein-based formula (Similac with Iron). All infants were given the formula within the first 24 hours of life, and were fed the study formula through the first year of life. Weight, length, and head circumference were recorded; bone mineral content (BMC) and bone width (BW) were measured at one-third distal length (midshaft) of the left radius and ulna during the first 18 days of life, and again at 6 weeks, and 3, 6, 9, and 12 months postnatally. Baby food was provided to all infants, and except for type of formula, food intake was similar for all infants. Weight, length, and head circumference were normal in all infants and not different between groups. BMC and BMC/BW were similar for both groups at entry into the study. Group 1 infants had significantly lower BMC and BMC/BW at 3, 6, 9, and 12 months of age (P less than 0.05 to P less than 0.0001) compared with group 2 infants, but their values were similar to those in previously studied infants receiving human milk with vitamin D supplementation. The long-term implication of differences in bone mineralization in early infancy for long-term skeletal mineralization remains to be determined.